1. Field of the Invention
The present invention relates to a trans-reflective liquid crystal display device, more particularly to the trans-reflective liquid crystal display device in which the brightness may be improved by using sequential backlight in reflection mode and transmission mode.
2. Description of the Related Art
Generally, a liquid crystal display device LCD realizes an image by modulating the light emitted from a light source called a backlight disposed at the rear or one side of a liquid crystal display device panel. Because only a small quantity of the light passes through the liquid crystal display panel, the power consumption of the backlight may be increased.
In order to decrease the power consumption of LCD displays, a reflective liquid crystal display device has been introduced. In the reflective liquid crystal display device, there is no need for a backlight, because external light is used to display the images. This means the power consumption may be decreased in the reflective liquid crystal display device and as a result, this device may be suitable for portable display devices.
The reflective liquid crystal display device is provided with reflective opaque materials to reflect the external light.
Because external light such as natural light or artificial light is not always available, the reflective liquid crystal display device may be used only where natural light or artificial light is available.
Accordingly, to overcome this problem, a trans-reflective liquid crystal display device having the advantages of transmissive and reflective liquid crystal display devices has been studied. The trans-reflective liquid crystal display device may be used as a transmission mode display device or a reflection mode display device.
Generally, because the trans-reflective liquid crystal display device has transmission and reflection modes, it is possible to use both the backlight and the external light as a light source. It has the advantage of reducing the power consumption in the trans-reflective liquid crystal display device as compared to a transmissive backlight liquid crystal display.
Referring to FIG. 1, a related art trans-reflective liquid crystal display device includes a thin film transistor TFT array substrate 105 on which a plurality of switching elements such as TFTs are formed, a color filter substrate on which color filters are formed, and a liquid crystal layer interposed between the TFT array substrate 105 and color filter substrate.
The TFT array substrate 105 includes a plurality of TFTs, formed on a first transparent substrate 100. The TFTs are each arranged in a pixel to apply a voltage signal to the liquid crystal layer. On the TFT array substrate 105, a gate insulator layer 110, an organic layer 120, a reflection electrode 130, a passivation layer 140 and a pixel electrode 150 are formed sequentially. A backlight is disposed to the rear of the TFT array substrate 105 or under the TFT array substrate 105 to emit the light to the TFT array substrate. Each pixel formed in the trans-reflective LCD panel may be divided into a reflection region and a transmission region. At this time, the reflection electrode is formed only in the reflection region.
The color filter substrate 165 includes a color filter 180 on the second transparent substrate 160 to display color when the light passes through the color filter layer 180. The color filter layer 180 comprises red, green or blue sub layers which are divided by a black matrix (not shown) from each other.
In the transmission mode, the light emitted from the backlight passes sequentially through the TFT array substrate 105, liquid crystal layer 190 and the color filter substrate 165 as shown by a solid line arrow in the FIG. 1.
In the reflection mode, the external light incident on the color filter substrate 165 passes through the liquid crystal layer 190 and then is reflected by the reflection electrode 130. Therefore, the reflected light is then emitted through the liquid crystal layer 190 and the color filter substrate 165 as a dotted line arrow in FIG. 1.
The light passes through the color filter layer 180 twice in the reflection mode, and the light passes through the color filter layer only once in the transmission mode. Because the light path through the color filter 180 is different in the transmission region and reflection region, the color purity is also different in the transmission and reflection modes.
The nonuniformity of the color purity causes the color of the trans-reflective LCD device to deteriorate. This nonuniformity of the color purity may be compensated by a color filter layer with a transmission region having a thickness that is twice the thickness of the color filter layer in the reflection region.
There are two methods used for forming the color filter layer having a thickness difference in the transmission and reflection regions. One method is to form a transparent organic layer 170 in the reflection region on the second transparent substrate 160 and then to form the color filter layer 180 on the transparent organic layer 170 and transmission region. Thus, the thickness of the color filter layer in the transmission region may be increased according to the thickness as the organic layer 170.
The other method is to etch the second transparent substrate 160 in the transmission region and then to form the color layer on the second transparent substrate 160. By this etching, the thickness of the color filter layer in the transmission region may be increased based upon the depth of the etched substrate.
In the above methods, however, the fabricating process is complicated. In the transmission mode, the light has to pass through the thick color filter 180, so that the transmission ratio and the brightness is lower.
In addition, if the area of the transmission region is increased to improve the brightness in transmission mode, the brightness may be decreased in reflection mode.